Flexible wiring boards and processes for producing flexible wiring board

ABSTRACT

Flexible wiring boards showing a small electrical resistance and yet having a small width.  
     The flexible wiring board according to the invention has a first wiring film and a second wiring film. Since the first wiring film has a larger thickness than the second wiring film, the sectional area and electrical resistance of the first wiring film can be enlarged because of the larger film thickness even in a case where the first and second wiring films have almost the same width. Therefore, a high current can flow through the first wiring film, though the wiring film has a small width. As a result, a high density of the flexible wiring board can be easily achieved.

TECHNICAL FIELD TO WHICH THE INVENTION BELONGS

[0001] This invention belongs to the technical filed of flexible wiringboards. More particularly, it relates to a technique of constructingflexible wiring boards having wiring films in which a high electriccurrent appropriately flows.

PRIOR ART

[0002] There have been frequently employed flexible wiring boards havingwiring films patterned in desired.

[0003] In FIG. 12, the numerical symbol 110 stands for an example offlexible wiring boards of the conventional art.

[0004] This flexible wiring board 110 has a first resin film 117, firstand second wiring films 121 and 122 located on the first resin film 117,and a second resin film 125 formed on the first and second wiring films121 and 122.

[0005] Although the first and second wiring films 121 and 122 has thesame thickness, the first wiring film 121 is wider than the secondwiring film 122. Therefore, the sectional area of the first wiring film121 is larger than the sectional area of the second wiring film 122.

[0006] Since the first wiring film 121 having the larger sectional areahas a smaller electrical resistance than the second wiring film 122, ahigher current can flows through it.

[0007] With the recent tendency toward high-density flexible wiringboards, it is required to be high density wiring film. However, suchwide first wiring film 121 as described above interferes the density ofwiring film.

[0008] The present invention, which has been made to solve the troublesencountering in the conventional art as described above, aims atproviding a technique of constructing flexible wiring boards havingwiring films which show a small electrical resistance and yet have beenfinely patterned.

SUMMARY OF THE INVENTION

[0009] To solve the above-described problems, the present inventionprovides a flexible wiring board having a first resin film, a firstwiring film the bottom face of which is embedded into the first resinfilm, and a second wiring film the bottom face of which is adhered tothe surface of the first resin film.

[0010] The present invention further provides a wiring board wherein thesurfaces of the first wiring films is flush with second wiring.

[0011] The present invention further provide a flexible wiring boardwherein a second resin film is formed on the surfaces of the first andsecond wiring films.

[0012] The present invention further provides a flexible wiring boardwherein at least one first opening is formed in the part of the secondresin film where the first wiring film is disposed.

[0013] The present invention further provides a flexible wiring boardwherein at least one second opening is formed in the part of the secondresin film where the second wiring film is disposed.

[0014] The present invention further provides a flexible wiring boardwherein at least one third opening is formed in the part of the firstresin film where the first wiring film is disposed.

[0015] The present invention further provides a flexible wiring boardwherein a metal bump is located in at least one of the first openings.

[0016] The present invention further provides a flexible wiring boardwherein a metal bump is located in at least one of the second openings.

[0017] The present invention further provides a flexible wiring boardwherein a metal bump is located in at least one of the third openings.

[0018] The present invention further provides a process for producing aflexible wiring board which comprises the step of etching a metal foilhalfway in the thickness direction to form concavities of apredetermined pattern on the surface of the metal foil, the step offorming a first resin film on the surface of the metal foil in the sidewhere the concavities are formed, the step of forming a resist layer,which is provided with openings at the parts corresponding to theconcavities, on the back face of the metal foil, and the step of etchingthe parts provided with the openings of the metal foil so as to dividethe first wiring film having the same thickness as the thickness of thepart of the metal foil where no concavity is formed and the secondwiring film having the same thickness as the thickness of the part wherethe concavities are formed.

[0019] The present invention further provides a process for producing aflexible wiring board wherein a second resin film is formed on the firstresin film in the side where the first and second wiring films arelocated.

[0020] The present invention further provides a process for producing aflexible wiring board wherein an opening is formed in the part of thefirst resin film where the first wiring film is located.

[0021] The present invention further provides a process for producing aflexible wiring board wherein openings are formed either in one or bothof the part of the second resin film where the first wiring film islocated and the part where the second wiring film is located.

[0022] The present invention has the constitution as described above.The flexible wiring board according to the present invention has thefirst and second wiring films separated from each other by a groove. Thefirst wiring film is embedded into the first resin film and, therefore,has a thickness larger by the embedded depth than the film thickness ofthe second wiring film. Since the section area of the first wiring filmis larger because of the larger film thickness, the electricalresistance of the first wiring film can be lowered even in case wherethe first wiring film has the same width as the second wiring film.

[0023] In the process for producing the flexible wiring board accordingto the present invention, concavities of a predetermined pattern areformed in a metal foil and then a resin solution is applied to the facehaving these concavities, thereby filling the concavities with the resinsolution. Subsequently, the whole construct is dried and baked. Thus afirst resin film having a flat surface can be formed.

[0024] When the above-described metal foil is patterned and a resinsolution is applied to the surface of each wiring films thus formed,grooves located among these wiring films are filled with the resinsolution. After drying and baking, a second resin film having a flatsurface can be formed.

[0025] The surfaces of the first is flush with the second wiring film.When the above-described second resin film is formed on the first andsecond wiring films, the thickness of the second resin film on the firstwiring film is almost the same as the thickness of the second resin filmon the second wiring film. By forming openings in the second resin filmby etching, therefore, the surfaces of the first and second wiring filmscan be exposed at the bottom faces of the openings almost at the sametime as the etching proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIGS. 1(a) to (e) are diagrams which illustrate the early stageof a process for producing the flexible wiring board of the firstexample according to the present invention.

[0027] FIGS. 2(f) to (j) are diagrams which illustrate the intermediatestage of a process for producing the flexible wiring board of the firstexample according to the present invention.

[0028] FIGS. 3(k) to (m) are diagrams which illustrate the latter stageof a process for producing the flexible wiring board of the firstexample according to the present invention.

[0029] FIGS. 4(a) to (c) are diagrams which illustrate the early stageof a process for producing a multi-layer flexible wiring board by usingthe flexible wiring board according to the present invention.

[0030]FIG. 5(d) is a diagram which illustrates the latter stage of aprocess for producing a multi-layer flexible wiring board by using theflexible wiring board according to the present invention.

[0031]FIG. 6 is a diagram which shows the relative location of the firstwiring film of the flexible wiring board of the present invention andthe copper wires of another flexible wiring board.

[0032]FIG. 7 is a diagram which illustrates the flexible wiring board ofthe second example according to the present invention.

[0033]FIG. 8 is a diagram which illustrates the flexible wiring board ofthe third example according to the present invention.

[0034]FIG. 9 is a diagram which illustrates the flexible wiring board ofthe fourth example according to the present invention.

[0035]FIG. 10 is a diagram which illustrates another example of theflexible wiring board according to the present invention.

[0036] FIGS. 11(a) and (b) are diagrams which illustrate another processfor connecting the flexible wiring board according to the presentinvention to another flexible wiring board.

[0037]FIG. 12 is a diagram which shows a flexible wiring board of theconventional art.

MODE FOR CARRYING OUT THE INVENTION

[0038] Now, the invention will be illustrated by reference to theattached drawings.

[0039] FIGS. 1(a) to (e), FIGS. 2(f) to (j) and FIGS. 3(k) to (m) show aproduction process of the flexible wiring board of the first exampleaccording to the present invention.

[0040] As FIG. 1(a) shows, a protective film 13 is applied to a metalfoil 11 (a copper foil of 32 μm in thickness is used herein). Afterforming a resist layer on the back face of the metal foil 11, the resistlayer is exposed, developed and pattered.

[0041] In FIG. 1(b), the numerical symbol 15 stands for the resist layerwhich has been patterned in the above step. A long and narrow opening 16is formed in this resist layer 15 by the patterning and the metal foil11 is exposed at the bottom face of this opening 16.

[0042] Next, the whole construct is immersed in a liquid etchant andthus the metal foil 11 exposed at the bottom face of the opening 16 isetched by 20 μm. Thus, a concavity 40 of 20 μm in depth, which has thesame planar shape as the opening 16, is formed in the metal foil 11, asshown in FIG. 1(c).

[0043] As a result, a thick film part 11 a protected with the resistlayer 15 and a thin film part 11 b, where the concavity 40 is located,are formed in the metal foil 11. The thick film part 11 a has the samethickness as the original thickness of the metal foil 11. On the otherhand, the thin film part 11 b has a thickness reduced by the depth ofthe concavity 40, compared with the thickness of the thick film part 11a. In this case, the thin film part 11 b has a thickness of 12 μm.

[0044] Next, the resist layer 15 on the surface of the metal foil 11 andthe protective film 13 on the back face of the metal foil 11 are peeledoff (FIG. 1(d)) and a polyimide resin solution is applied to the surfaceof the metal foil 11 wherein the concavity 40 is formed until the thinfilm part 11 b and the thick film part 11 a are submerged. Thus theconcavity 40 in the metal foil 11 is filled with this resin solution.Then the construct is dried as such and baked. As a result, a firstresin film 17 made of a polyimide film with a flat surface is formed asshown in FIG. 1(e).

[0045] Next, a protective film 27 is applied to the flat surface of thefirst resin film 17 and a resist layer is formed on the face in theopposite side of the metal foil 11 to the side where the first resinfilm 17 has been formed. Subsequently, the resist layer is patterned byexposing and developing to thereby form a resist layer 18 having pluralopenings 19 on the thin film part 11 b of the metal foil 11, as shown inFIG. 2(f). The thin film part 11 b of the metal foil 11 is exposed atthe bottom faces of the openings 19.

[0046] These openings have a long and narrow shape and the resist layer18 is divided into plural parts by these openings 19. The numericalsymbol 18 b stands for the resist layer located on the surface of thethick film part 11 a of the metal foil 11, while the numerical symbol 18a stands for the resist layer located on the surface of the thin filmpart 11 b of the metal foil 11.

[0047] Next, the whole construct is immersed in a liquid etchant andthus the metal foil 11 is etched in the parts located at the bottomfaces of the openings 19. After etching the metal foil 11 by thethickness of the thin film part 11 b, the resist layer 18 is peeled off.

[0048]FIG. 2(g) shows the state thus achieved. In this figure, thenumerical symbol 41 stands for a groove formed in the metal foil 11 byetching. The groove 41 has the same planar shape as the opening 19 ofthe resist layer 18. The first resin film 17 is exposed at the bottomface of the groove 41 and the metal foil 11 is divided into plural partsby these grooves 41.

[0049] In FIG. 2(g), the numerical symbol 21 stands for the first wiringfilm consisting of the part of the metal foil 11 where the thick filmpart 11 a is located, while the numerical symbol 22 stands for thesecond wiring film consisting of the part of the metal foil 11 where thethin film part 11 b is located.

[0050] In this state, the bottom faces of the first wiring film 21 andthe second wiring film 22 are fixed to the first resin film 17. Thebottom face of the second wiring film 22 is located on the surface ofthe first resin film 17, while the bottom face of the first wiring film21 is located more deeply than the bottom face of the second wiring film22 and embedded into the first resin film 17. The surface of the firstwiring film 21 is located at the same height as the surface of thesecond wiring film 22 and thus the surfaces of first wiring film 21 isflush with the surface of the second wiring film 22. Therefore, thefirst wiring film 21 has a thickness larger than the second 22 by thedepth embedded into the first resin film 17.

[0051] Next, a polyimide resin solution is applied to the surface of thefirst resin film 17 in the side where the first and second wiring films21, 22 is formed until the first and second wiring films 21 and 22 aresubmerged. Thus the groove dividing the first and second wiring films 21and 22 is filled with the resin soluiton. Then the whole construct isdried and baked. As a result, a second resin film 25 made of a polyimidefilm having a flat surface is formed (FIG. 2(h)).

[0052] Next, a resist layer patterned in a predetermined shape is formedon the surface of the second resin film 25. In FIG. 2(i), the numericalsymbol 28 stands for a resist layer 28 formed on the surface of thesecond resin film 25. In this resist layer 28, circular openings areformed at predetermined positions by patterning. In FIG. 2(i), thenumerical symbol 29 a stands for a circular opening formed in the partwhere the first wiring film 21 is located, while the numerical symbol 29b stands for a circular opening formed in the part where the secondwiring film 22 is located. The second resin film 25 is exposed at thebottom faces of these circular openings 29 a and 29 b.

[0053] Subsequently, an alkali solution is sprayed onto the surface ofthe resist layer 28 so as to etch the second resin film 25 exposed atthe bottom faces of the circular openings 29 a and 29 b in the resistlayer 28.

[0054] The thickness of the second resin film 25 on the first wiringfilm 21 is almost the same as the thickness of the second resin film 25on the second wiring film 22. Therefore, the surfaces of the first andsecond wiring films are exposed almost at the same time as the etchingproceeds.

[0055] Next, the resist layer 28 is removed. As a result, first andsecond openings 42 a and 42 b, which have been patterned so as to givethe same diameter as the circular openings 29 a and 29 b, are formed inthe second resin film 25 located upper side of the first wiring film 21and the second wiring film 22, as FIG. 2(j) shows.

[0056] After peeling off the protective film 27 from the surface of thefirst resin film 17, a resist layer 37 having circular openings 39 atpredetermined positions of the first resin film 17 located below thefirst wiring film 21 is formed. Then a new protective film 30 is appliedto the surface of the second resin film 25 (FIG. 3(k)).

[0057] Subsequently, an alkali solution is sprayed onto the surface ofthe resist layer 37 so as to etch the first resin film 17 exposed at thebottom face of the circular opening 39.

[0058] Next, the protective film 30 and the resist layer 37 are peeledoff. As a result, a third opening 43, which has been patterned so as togive the same diameter as the circular opening 39, is formed in thefirst resin film located upper side of the first wiring film 21 as shownin FIG. 3(l).

[0059] Next, the whole construct is immersed in an electrolytic platingsolution and electric current flows therethrough. Thus, a thin metalfilm grows at the bottom faces of the first, second and third openings42 a, 42 b and 43 to form a metal coating 45.

[0060] In FIG. 3(m), the numerical symbol 10 stands for the flexiblewiring board according to the present invention in the state after theformation of the metal coating 45.

[0061] In this flexible wiring board 10, the width of the first wiringfilm 21 is almost the same as the width of the second wiring film 22.However, the thickness of the first wiring film 21 is larger than thethickness of the second wiring film 22 and thus the first wiring film 21has a larger sectional area because of the larger film thickness.

[0062] Next, a process for producing a multi-layer flexible wiring boardby laminating the flexible wiring board 10 constructed by the aboveprocess on other flexible wiring boards will be illustrated by referenceto FIGS. 4(a) to (c) and 5(d).

[0063] The numerical symbol 50 ₁ in FIG. 4(a) and the numerical symbol50 ₂ in FIG. 4(c) stand for monolayer flexible wiring boards to be usedin laminating on the flexible wiring board according to the presentinvention. Members common to these flexible wiring boards 50 ₁ and 50 ₂are discriminate from each other by attached characters 1 and 2respectively.

[0064] These flexible wiring boards 50 ₁ and 50 ₂ respectively have basefilms 51 ₁ and 51 ₂, plural copper wires 53 ₁ and 53 ₂ located on thesebase films 51 ₁ and 51 ₂, and cover films 55 ₁ and 55 ₂ formed on thecopper wires 53 ₁ and 53 ₂. The cover films 55 ₁ and 55 ₂ include athermoplastic resin as the main component.

[0065] On the surfaces of the copper wires 53 ₁ and 53 ₂, metal bumps 59₁ and 59 ₂ are formed upright and the heads of these metal bumps 59 ₁and 59 ₂ protrude respectively on the surfaces of the cover films 55 ₁and 55 ₂.

[0066] To construct a multi-layer flexible wiring board by laminatingthe above-described flexible wiring boards 50 ₁ and 50 ₂ on the flexiblewiring board 10 according to the present invention, the metal bumps 59 ₁of one of the monolayer flexible wiring board 50 ₁ are located towardthe first and second openings 42 a and 42 b in the second resin film 25of the flexible wiring board 10 according to the present invention, asshown in FIG. 4(a).

[0067] Next, these metal bumps 59 ₁ are brought into contact with themetal coating 45 exposed at the bottom faces of the first and secondopenings 42 a and 42 b. Thus, the second resin film 25 of the flexiblewiring board 10 and the cover film 55 ₁ of the flexible wiring board 50₁ are closely contact with each other.

[0068] When the whole construct is pressed in this state under heating,the thermoplastic resin in the cover film 55 ₁ exerts its adhesivenessdue to the heating. When the whole construct is further heated, a solderplating layer (not shown) formed on the surface of the metal bumps 59 ₁is molten.

[0069] Subsequently, the whole construct is cooled. Then the moltensolder plating layer solidifies. Thus, metal bumps 59 ₁ and the firstand second wiring films 21 and 22 are connected to each other via thesolder plating layer which has solidified. At the same time, theflexible wiring boards 10 and 50 ₁ are mechanically laminated via thecover film 55 ₁.

[0070] Next, the metal bumps 59 ₂ of the other monolayer flexible wiringboard 50 ₂ are located toward the third openings 43 of the first resinfilm 17 of the flexible wiring board 10 according to the presentinvention.

[0071] Next, these metal bumps 59 ₂ are brought into contact with themetal coating 45 exposed at the bottom face of the third openings 43 asshown in FIG. 4(c). Thus, the first resin film 17 is closely contactwith the cover film 55 ₂. When the whole construct is pressed in thisstate under heating and then cooled, the metal bumps 59 ₂ are connectedto the first wiring film 21 via a solder plating layer (not shown) and,at the same time, the flexible wiring boards 10 and 50 ₂ are laminatedvia the cover film 55 ₂.

[0072] The numerical symbol 5 in FIG. 5(d) shows a multi-layer flexiblewiring board constructed by laminating monolayer flexible wiring boards50 ₁ and 50 ₂ on respective faces of the flexible wiring board 10according to the present invention.

[0073] In this flexible wiring board 5, the copper wires 53 ₁ and 53 ₂are connected to the first and second wiring films 21 and 22 via themetal bumps 59 ₁ and 59 ₂.

[0074]FIG. 6 is a diagram showing the relative location of the firstwiring film 21 and copper wires 53 ₁ of another flexible wiring board 50₁. In this diagram, three copper wires 53 ₁ are shown. Each copper wire53 ₁ has a wide part, on the surface of which a metal bump 59 ₁ isprovided, and a narrow part.

[0075] Each metal bump 59 ₁ is connected to the metal coating 45 on thefirst wiring film 21 via a plating layer (not shown). Namely, the firstwiring film 21 is electrically connected to plural copper wires 53 ₁.

[0076] Because of having a larger thickness than each copper wire 53 ₁the first wiring film 21 has a larger sectional area. Owing to thelarger sectional area, the electrical resistance of the first wiringfilm 21 is smaller than the electrical resistance of each copper wire 53₁. In case where an electrical current flows from plural copper wires 53₁ into the first wiring film 21, the first wiring film 21 can allow thepassage of the current with a small voltage drop.

[0077] Although illustration has been made on a case where the metalcoating 45 is exposed at the bottom faces of the first to third openings42 a, 42 b and 43 of the flexible wiring board 10, the present inventionis not restricted thereto.

[0078] The numerical symbols 70 in FIG. 7, 80 in FIG. 8 and 90 in FIG. 9respectively show flexible wiring boards of the second to fourthexamples according to the present invention.

[0079] Similar to the flexible wiring board 10 as shown in FIG. 3(m),these flexible wiring boards 70, 80 and 90 respectively have first resinfilms 76, 86 and 96, first and second wiring films 71, 72, 81, 82, 91and 92 formed on the first resin films 76, 86 and 96, and second resinfilms 77, 87 and 97 formed on the surface of the first and second wiringfilms 71, 72, 81, 82, 91 and 92. The first wiring films 71, 81 and 91are separated from the second wiring films 72, 82 and 92 by grooves 73,83 and 93.

[0080] Third openings 78, 88 and 98 are formed in the first resin films76, 86 and 96, while first and second openings 74 a, 74 b, 84 a, 84 b,94 a and 94 b are formed in the second resin films 77, 87 and 97.

[0081] In the flexible wiring board 70 of the second example as shown inFIG. 7, among these flexible wiring boards 70, 80 and 90, a metalcoating 75 formed at the bottom surface of the first wiring film 71 isexposed at the bottom face of the third opening 78, while metal bumps 79are provided in the first and second openings 74 a and 74 b. These metalbumps 79 are formed upright respectively on the surface of first andsecond wiring films 71 and 72. The head of each metal bum 79 protrudeson the surface of the second resin film 77.

[0082] In the flexible wiring board 80 of the third example as shown inFIG. 8, a metal coating 85 formed at the surfaces of the first andsecond wiring films 81 and 82 is exposed at the bottom faces of thefirst and second openings 84 a and 84 b, while metal bumps 89 areprovided in the third opening 88. These metal bumps 89 are formedupright on the bottom surface of the first wiring film 71. The head ofeach metal bump 89 protrudes on the surface of the first resin film 86,similar to the case of the flexible wiring board 70 of the secondexample.

[0083] In the flexible wiring board 90 of the fourth example as shown inFIG. 9, metal bumps 99, which are formed upright on the surfaces of thefirst and second wiring films 91 and 92, are provided in the first andsecond openings 94 a and 94 b. The head of each metal bump 99 protrudeson the surface of the second resin film 97. In the third opening 98, ametal bump 99, which is formed upright on the bottom face of the firstwiring film 91, is provided and the head of the metal bump 99 protrudeson the surface of the first resin film 96.

[0084] Metal coatings 75, 85 and 95 are formed respectively on the headsof the metal bumps 79, 89 and 99 of these flexible wiring boards 70, 80and 90 of the second to fourth examples.

[0085] Although illustration has been made on a case where the flexiblewiring board according to the present invention is connected to otherflexible wiring boards 50 to construct the multi-layer flexible wiringboard 5, the present invention is not restricted thereto.

[0086] The numerical symbol 6 in FIG. 10 stands for a multi-layerflexible wiring board constructed by connecting the flexible wiringboards according to the present invention represented by 70 in FIG. 7and 80 in FIG. 8 to the flexible wiring board 10 according to thepresent invention represented by 10 in FIG. 3(m). Namely, it is alsopossible to obtain the multi-layer flexible wiring board 6 by using theflexible wiring boards 10, 70 and 80 of the present invention.

[0087] Use may be made of a resin film containing a thermoplastic resinas at least one of the first and second resin films of the flexiblewiring board according to the present invention. In this case, it isunnecessary to use a thermoplastic resin in the resin films of the otherflexible wiring board to be laminated.

[0088] Moreover, a case where none of the resin films used in theflexible wiring boards to be laminated has thermoplasticity also fallswithin the scope of the present invention.

[0089] The numerical symbol 50 in FIG. 11 stands for a flexible wiringboard which has the same structure as the flexible wiring boardsrepresented by 50 ₁ and 50 ₂ in FIG. 4 but the resin constituting thecover film 55 has no thermoplasticity.

[0090] To connect this flexible wiring board 50 to the flexible wiringboard 10 according to the present invention, a thermoplastic resin film23 (i.e., an adhesive film) is put between the flexible wiring boards 10and 501 and then positioning is performed so that the metal bumps 59 arelocated toward the first and second openings 42 a and 42 b (FIG. 11(a)).

[0091] Next, the resin film 23 is inserted between the flexible wiringboards 10 and 50. Thus, the heads of the metal bumps 59 are brought intocontact with the resin film 23. Then the whole construct is pressedunder heating in this state. As a result, the resin film is softened byheating and the metal bumps 59 push aside the softened resin film 23 dueto the pressure. Thus the heads of the metal bumps 59 are brought intocontact with the metal coating 45 at the bottom faces of the first andsecond openings 42 a and 42 b, thereby electrically connecting theflexible wiring boards 10 and 50 to each other.

[0092] In this process, the resin film exhibits adhesiveness underheating. Thus, the flexible wiring boards 10 and 50 are mechanicallyconnected to each other too so as to give a multi-layer flexible wiringboard 8 (FIG. 11(b)). It is also possible to use a so-called anisotropicconductive film, wherein conductive particles are dispersed in a resinfilm, as a substitute for the resin film 23.

[0093] Although illustration has been made on a case where solder metallayers are formed on the heads of the metal bumps 59 of the monolayerflexible wiring boards 50 ₁ and 50 ₂ to be used in laminating to theflexible wiring board 10 according to the present invention, the presentinvention is not restricted thereto.

[0094] For example, a flexible wiring board having no solder platinglayer on the heads of the metal bumps can be laminated to the flexiblewiring board 10 according to the present invention by, aftermechanically connecting to the flexible wiring board 10 by the steps ofFIGS. 4(a) and (b), applying ultrasonic wave so as to connect the metalbumps to the wiring films.

[0095] The present invention makes it possible to construct wiring filmshaving a small width and yet showing a small electrical resistance. Inflexible wiring boards provided with such wiring films, a high densitycan be easily achieved.

What is claimed is:
 1. A flexible wiring board having: a first resinfilm; a first wiring film the bottom face of which is embedded into saidfirst resin film; and a second wiring film the bottom face of which isin contact with the surface of said first resin film.
 2. The flexiblewiring board as claimed in claim 1 wherein a surfaces of said firstwiring film is flush with said second wiring film.
 3. The flexiblewiring board as claimed in claim 1 wherein a second resin film is formedon the surfaces of said first and second wiring films.
 4. The flexiblewiring board as claimed in claim 3 wherein at least one first opening isformed in the part of said second resin film where said first wiringfilm is disposed.
 5. The flexible wiring board as claimed in claim 3wherein at least one second opening is formed in the part of said secondresin film where said second wiring film is disposed.
 6. The flexiblewiring board as claimed in claim 1 wherein at least one third opening isformed in the part of said first resin film where said first wiring filmis disposed.
 7. The flexible wiring board as claimed in claim 4 whereina metal bump is located in at least one of said first openings.
 8. Theflexible wiring board as claimed in claim 5 wherein a metal bump islocated in at least one of said second openings.
 9. The flexible wiringboard as claimed in claim 6 wherein a metal bump is located in at leastone of said third openings.
 10. A process for producing a flexiblewiring board which comprises: the step of etching a metal foil halfwayin the thickness direction to form concavities of a predeterminedpattern on the surface of said metal foil; the step of forming a firstresin film on the surface of said metal foil in the side where saidconcavities are formed; the step of forming a resist layer, which isprovided with openings at the parts corresponding to the concavities, onthe back face of said metal foil; and the step of etching the parts ofsaid metal foil provided with said openings so as to devide the firstwiring film having the same thickness as the thickness of the part ofsaid metal foil where no concavity is formed and the second wiring filmhaving the same thickness as the thickness of the part where saidconcavities are formed.
 11. The process for producing a flexible wiringboard as claimed in claim 10 wherein a second resin film is formed onsaid first resin film in the side where said first and second wiringfilms are located.
 12. The process for producing a flexible wiring boardas claimed in claim 10 wherein an opening is formed in the part of saidfirst resin film where said first wiring film is disposed.
 13. Theprocess for producing a flexible wiring board as claimed in claim 11wherein openings are formed either in one or both of the part of saidsecond resin film where said first wiring film is located and the partwhere said second wiring film is disposed.